Rate vs. Time Graph for a 1st order Reaction

[mlb2358]

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In TBR's overview of kinetics, they show a graph of rate vs. time for a first order reaction that is linear and has a negative slope. They also provide a chart of time and concentration values. If we define rate as change in concentration/change in time and graph these values, I don't get a straight line. I am wondering what I am doing wrong here. I will attach an image showing the graph and chart for reference.

Thanks

 

[Cawolf]

For a first order reaction to produce the graph in the second figure you need to plot the ln [A] ( y) vs time (x).

The slope of the graph will be -k.